With the aid of modern imaging methods, three-dimensional image data is often created which can be used for visualizing an imaged object under investigation and for other uses. For example, organs or other structures of a patient or other object under investigation can be recorded in the spatial position thereof in order, for example, to initiate further analyses based thereon. In particular, a “segmentation” of individual organs or structures can be carried out so that this data can be used for subsequent considerations. It is herein decisive that geometrical distortions during the generation of the three-dimensional image data, i.e. the “volume image data” are suppressed or prevented as far as possible.
The projection measurement data needed for the reconstruction of the volume image data is acquired, for example, by way of a computed tomography (CT) system. In CT systems, a combination of an X-ray source and, mounted opposite thereto, an X-ray detector arranged on a rotating gantry revolves around a measurement space in which the object under investigation (which is identified below as a patient, but without restricting the generality thereof) is situated. The center of rotation (also known as “isocenter”) coincides with a “system axis” z which extends parallel to an advancing direction of a patient table. With the aid of the patient table, the patient can be moved in and out of the measurement space. During one or two rotations, the patient is irradiated with X-ray radiation from the X-ray source, wherein projection measurement data or X-ray projection data is detected with the aid of the X-ray detector positioned opposite thereto. The circulation of the X-ray system defines an “axial plane” or a “transverse plane” in which projection data is generated and recorded in each case and which is transverse, in particular perpendicular, to the system axis z. The spatial directions within an axial plane of this type are identified below as “axial plane directions”.
In modern computed tomography, in addition to the axial representation, the reconstructed (two-dimensional) image data or sectional images are typically reformatted into different spatial directions or from these, as aforementioned, coherent three-dimensional image data, that is, “volume image data” is generated and visualized. However, the aforementioned further processing of the reconstructed volume image data, for example, for segmentation or for three-dimensional representation, is relatively difficult. The reconstructed volume image data depends at least partially on the geometry of the X-ray detector or, in some other way, on the design or the “hardware” of the computed tomography system. The design of the detector and of the computed tomography system pre-determines, for example, a minimum achievable resolution width of the three-dimensional image data, that is, the minimal size of a voxel.
In particular, mostly, axial “primary layers” which have a specific extent in the z-direction are reconstructed on the basis of the projection measurement data acquired in an axial plane. This extent of the primary layers can be pre-determined by the detector geometry and, in particular, by the extent of an individual detector element (pixel of the detector) in the direction of the system axis z. The primary layers therefore have a “hardware-dependent extent” in the z-direction. As mentioned, the patient table or the gantry is displaced parallel to the system axis z so that the detector and the X-ray source are moved relative to the patient. From the series of several axial sectional images or primary layers, three-dimensional image data can also be reconstructed, wherein the extent of a volume image point (voxel) is less than the extent of a detector element in the z-direction. For this purpose, the advance of the patient table in the z-direction (i.e. the distance covered by the detector in the z-direction between two recording time points from the same projection direction) must only be less than the extent of a detector element in the z-direction. In this case, projection measurement data or primary layers are produced which overlap spatially in the z-direction, and which allow a reconstruction with increased resolution in the z-direction; in this regard, a resolution increase by “overscanning” can be said to take place. Furthermore, corresponding volume image data has a “hardware-dependent extent” in the z-direction which is given by the advance of the patient table and the size of individual detector elements.
The resolution of the image data in the axial plane directions, that is, transversely to the system axis z is also “hardware-dependently pre-determined”. This essentially means that the resolution of the primary layers in the axial plane is determined by the geometry of the measurement space and the circulation of the radiation source around the measurement space.
Since these “hardware-dependent factors” can vary even in different CT scans with the same CT system (for example, the advancing speed of the patient table can be changed), the aforementioned difficulties often arise in the further use of the reconstructed image data.